1. Field of the Invention
The present invention relates to an optical transmission device including a Mach-Zehnder interferometer (hereinafter referred to as an “MZ interferometer”), and more particularly to an optical transmission device capable of solving the problem of unnecessary residual sideband components to thereby obtain a high-quality optical intensity-modulated signal in a case where the optical modulation section for performing a single-sideband modulation (hereinafter referred to as “SSB”), a single-sideband suppressed-optical carrier modulation (hereinafter referred to as “SSB-SC”), etc., has a manufacturing error and wavelength dependence.
2. Description of the Background Art
In a system for realizing wireless communications by using high-frequency signals such as those in the microwave band and the millimeter wave band, it has been seen as a promising method to employ an optical transmission scheme using an optical fiber, which has a wide transmission band and little loss, for exchanging signals between a control station and a base station. Moreover, it is expected that the frequency band for use in wireless communications will transition from the microwave band to the millimeter wave band. Accordingly, there is an increasing demand for optical transmission devices for use in the millimeter wave band.
FIG. 14 is a schematic diagram showing an optical fiber network between a control station and base stations in a system for realizing wireless communications by using high-frequency signals. As shown in FIG. 14, the optical fiber network includes a control station 110 for transmitting an optical signal, an optical fiber 300 for carrying an optical signal, and base stations 200-1, . . . , 200-n for receiving an optical signal and realizing wireless communications with, for example, portable terminals (not shown).
The control station 110 includes an optical transmission device 120. The optical transmission device 120 transmits an optical signal that has been modulated with a high-frequency signal such as a microwave signal or a millimeter wave signal. The transmitted optical signal travels through the optical fiber 300 so as to be photoelectrically converted at a light-receiving section (not shown) of each of the base stations 200-1, . . . , 200-n. The photoelectrically converted signal is transmitted to a portable terminal, or the like, as a wireless signal of a high-frequency band such as the microwave band or the millimeter wave band.
However, where an optical signal having a wavelength of 1.55 μm, for example, is transmitted along a single-mode optical fiber that is for use with an optical signal having a wavelength of 1.31 μm, the transmitted signal periodically attenuates by a certain level over every passage of a particular transmission distance. The attenuation phenomenon occurs as the upper and lower sideband components, which are produced when the optical carrier is intensity-modulated with a high-frequency signal, are influenced by the wavelength dispersion.
FIG. 15 is a diagram showing a frequency spectrum of an optical intensity-modulated signal, which is produced by performing an optical intensity modulation so as to include the optical carrier component, the upper sideband component and the lower sideband component. Where an optical carrier having a frequency f0 is intensity-modulated with a high-frequency signal having a frequency f1, the upper sideband component (frequency f0+f1) appearing on the higher-frequency side of the optical carrier and the lower sideband component (frequency f0-f1) appearing on the lower-frequency side of the optical carrier have frequencies that are apart from each other by 2×f1, i.e., twice the frequency of the modulation signal. The upper sideband component and the lower sideband component at the point where the optically-modulated signal is transmitted each have a predetermined phase shift angle with respect to the carrier component, which are, for example, +π/4 and −π/4 (rad), respectively.
However, as the signal transmission distance increases, the phase shift angle changes to periodically become +π/2 and −π/2 (rad). At points where the phase shift angle with respect to the carrier component is +π/2 and −π/2 (rad), the phase of the upper sideband component is shifted by π (rad) from that of the lower sideband component. When an optical intensity-modulated signal is photoelectrically converted by the receiver at such a point, the beat component f1 between the optical carrier and the upper sideband and the beat component f1 between the optical carrier and the lower sideband cancel each other due to interference. As a result, the modulated signal, which has been photoelectrically converted, disappears.
FIG. 16 is a diagram showing the relationship between the received signal power on the optical signal receiving side and the transmission distance. Referring to FIG. 16, the disappearance of the received signal power occurs at regular intervals of a particular transmission distance. This means that the modulated signal, which has been photoelectrically converted, disappears at regular intervals of a particular transmission distance. The particular transmission distance decreases as the frequency of the modulated signal increases. Accordingly, where the modulated signal is a millimeter wave signal, the modulated signal disappears frequently at intervals of a short transmission distance, which substantially hinders the optical transmission.
One possible method to avoid such a hindrance in an optical fiber network as shown in FIG. 14 is such that an optical carrier is intensity-modulated with a microwave signal by the control station 110, and the optical signal is received by the base stations 200-1, . . . , 200-n so that the received optical signal is up-converted to a millimeter wave signal. With such a method, however, it is necessary to provide an up-converter in each of the base stations 200-1, . . . , 200-n. This increases the size and cost of the base stations 200-1, . . . , 200-n, and also makes more difficult the maintenance thereof.
Thus, where the modulated signal is a millimeter wave signal, the disappearance of the modulated signal needs to be avoided without up-converting the signal at the receiver. Therefore, it has been discussed to employ, for example, SSB optical modulation (See Non-Patent Document 1).
Non-Patent Document 1: Graham H. Smith et., IEEE Transactions on Microwave Theory and Techniques, vol. 45, No. 8 p. 1410-1415, August, 1997.
FIG. 17 is a diagram showing a configuration of a conventional optical transmission device disclosed in Non-Patent Document 1. The optical transmission device corresponds to the optical transmission device 120 described above. Referring to FIG. 17, the optical transmission device includes a DFB (distributed feedback) laser 400 for outputting an optical carrier, an isolator 410 for receiving the optical signal from the DFB laser 400, a polarization controller 420 for receiving the optical carrier from the isolator 410, a signal generator 440 for outputting a signal to be transmitted, an amplifier 450 for receiving the signal from the signal generator 440, a branching unit 460 for receiving the signal from the amplifier 450, a phase adjustment section 470 for receiving the signal from the branching unit 460, and an MZ interferometer 430 for receiving the signal from the phase adjustment section 470, the signal from the branching unit 460 and the signal from the polarization controller 420.
The optical carrier outputted from the DFB laser 400 is inputted to the MZ interferometer 430 via the isolator 410 and the polarization controller 420. The isolator 410 is provided for preventing reflected light from entering the DFB laser 400. Typically, the isolator 410 is provided in the DFB laser 400. The polarization controller 420 is for adjusting the plane of polarization of the input light to a plane of polarization that is suitable for intensity modulation.
A high-frequency signal to be transmitted is outputted from the signal generator 440, and is amplified by the amplifier 450 to a predetermined level. The amplified signal is branched into two signals by the branching unit 460. One branch signal is inputted directly to the MZ interferometer 430. The other branch signal passes through the phase adjustment section 470 where the phase thereof is changed by π/2 (rad). The signal whose phase has been changed is inputted to the MZ interferometer 430.
FIG. 18A is a diagram showing the frequency spectrum of an optical intensity-modulated signal produced through an ideal SSB optical modulation of an optical carrier having a frequency f0 with a modulated signal having a frequency f1. When an optical carrier inputted to the MZ interferometer 430 is branched into two optical waveguides, and the phase of the high-frequency signal to be applied to one optical waveguide and that of the high-frequency signal to be applied to the other optical waveguide are shifted from each other by π/2 (rad), there is obtained an optical intensity-modulated signal having an upper or lower sideband component in addition to the optical carrier component, as shown in FIG. 18A. Such a method where an SSB optical modulation is performed by using two high-frequency signals (modulated signals) whose phases are different from each other by π/2 (rad) is known as a phase method.
Where an optical intensity-modulated signal having only the carrier component and one sideband component is transmitted over a long distance via an optical fiber, the beat component cancellation at the receiver does not occur because there is only one, upper or lower, sideband component, whereby the modulated signal, which has been photoelectrically converted, does not disappear. Therefore, by using an SSB optical modulation scheme, it is possible to avoid the influence of signal disappearance due to the wavelength dispersion.
FIG. 19 is a diagram showing a configuration of an ordinary MZ interferometer 430 capable of an SSB optical modulation. The MZ interferometer 430 branches the input light through a branching section 960 provided at the input end into two optical waveguides 940 and 950. A DC (direct current) bias voltage and an RF (radio frequency) signal voltage are applied to the optical waveguides 940 and 950, respectively, and the refractive indexes of the optical waveguides 940 and 950 change according to the voltages. Because of the changes in the refractive indices, the optical carriers passing through the optical waveguides 940 and 950 are each phase-modulated. An optocoupler section 970 provided at the output end of the MZ interferometer 430 combines together the light beams, which have been phase-modulated through the optical waveguides 940 and 950. By this combining, there is produced an optical intensity-modulated signal in which a modulated signal is superposed over an optical carrier.
However, the optical transmittance characteristics of the MZ interferometer 430 have problems of wavelength dependence and a manufacturing error (see, for example, Japanese Laid-Open Patent Publication No. 2005-208172 (hereinafter “Patent Document 1”)). Because of the wavelength dependence and the manufacturing error, the SSB characteristics deteriorate. Specifically, while it is ideal that the light branching section 960 of the MZ interferometer 430 evenly branches the carrier into two light beams of the same intensity, the branching ratio will not in practice be 1:1 due to the wavelength dependence and the manufacturing error of the MZ interferometer 430. FIG. 18B is a diagram showing the frequency spectrum of an optical intensity-modulated signal where the light branching ratio of the light branching section 960 is not 1:1 in the MZ interferometer 430 shown in FIG. 19. In an ideal state where the light branching ratio is 1:1, the frequency spectrum of the optical intensity-modulated signal is such that an unnecessary sideband component disappears as shown in FIG. 18A. Where the branching ratio is not 1:1, the frequency spectrum of the optical intensity-modulated signal is such that there is an unnecessary residual sideband component as shown in FIG. 18B.
The modulation operation of the MZ interferometer 430 will now be described below using numerical expressions. The input optical carrier is expressed by Expression (1).E(t)=E cos ω0t  Expression (1)
The branching ratio of the light branching section 960 is herein denoted as γ (γ<1). The branching ratio γ means (the intensity of one of light beams branched into one optical waveguide)/(the intensity of input light to the light branching section). In this case, the photoelectric field of the optical carrier passing through one of the optical waveguides can be represented by EA(t) in the first line of Expression (2). The photoelectric field of an optical intensity-modulated signal that can be obtained through an optical intensity modulation of the optical carrier can be represented by EA(t) in the fourth line of Expression (2). Herein, Va(t) is the RF (radio frequency) signal voltage applied to one optical waveguide, ka is the amplitude of the RF signal, and VA is the amount of phase to be given to the optical carrier by the DC bias voltage applied to one optical waveguide.
                                                                                          E                  A                                ⁡                                  (                  t                  )                                            =                              γ                ⁢                                                                  ⁢                E                ⁢                                                                  ⁢                cos                ⁢                                                                  ⁢                                  ω                  0                                ⁢                t                                                                                                                          V                  a                                ⁡                                  (                  t                  )                                            =                                                k                  a                                ⁢                cos                ⁢                                                                  ⁢                                  ω                  1                                ⁢                t                                                                                                        V                A                            =              0                                                                                                            E                  A                                ⁡                                  (                  t                  )                                            =                              γ                ⁢                                                                  ⁢                E                ⁢                                  {                                      cos                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                                              V                            a                                                    ⁡                                                      (                            t                            )                                                                          +                                                  V                          a                                                                    )                                                        }                                                                                                        =                              γ                ⁢                                                                  ⁢                E                ⁢                                  {                                      cos                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                                              k                            a                                                    ⁢                          cos                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                                    )                                                        }                                                                                                        =                                                                    γ                    ⁢                                                                                  ⁢                    E                                    2                                ⁡                                  [                                                                                                                                          cos                            ⁢                                                          (                                                                                                ω                                  0                                                                ⁢                                t                                                            )                                                        ⁢                                                          cos                              ⁡                                                              (                                                                                                      k                                    a                                                                    ⁢                                  cos                                  ⁢                                                                                                                                          ⁢                                                                      ω                                    1                                                                    ⁢                                  t                                                                )                                                                                                              -                                                                                                                                                              sin                          ⁢                                                      (                                                                                          ω                                0                                                            ⁢                              t                                                        )                                                    ⁢                                                      sin                            ⁡                                                          (                                                                                                k                                  a                                                                ⁢                                cos                                ⁢                                                                                                                                  ⁢                                                                  ω                                  1                                                                ⁢                                t                                                            )                                                                                                                                                            ]                                                                                                        =                                                                    γ                    ⁢                                                                                  ⁢                    E                                    2                                ⁡                                  [                                                                                                                                          cos                            ⁡                                                          (                                                                                                ω                                  0                                                                ⁢                                t                                                            )                                                                                ⁢                                                      {                                                                                                                            J                                  0                                                                ⁡                                                                  (                                  k                                  )                                                                                            -                                                                                                                                                                                                                                                                              2                              ⁢                                                                                                J                                  2                                                                ⁡                                                                  (                                                                      k                                    a                                                                    )                                                                                            ⁢                              cos                              ⁢                                                                                                                          ⁢                              2                              ⁢                                                              ω                                1                                                            ⁢                              t                                                        }                                                    -                                                                                                                                                                                          sin                            ⁡                                                          (                                                                                                ω                                  0                                                                ⁢                                t                                                            )                                                                                ⁢                                                      {                                                                                          2                                ⁢                                                                                                      J                                    1                                                                    ⁡                                                                      (                                                                          k                                      a                                                                        )                                                                                                  ⁢                                cos                                ⁢                                                                                                                                  ⁢                                                                  ω                                  1                                                                ⁢                                t                                                            -                                                                                                                                                                                                                                                2                            ⁢                                                                                          J                                3                                                            ⁡                                                              (                                                                  k                                  a                                                                )                                                                                      ⁢                            cos                            ⁢                                                                                                                  ⁢                            3                            ⁢                                                          ω                              1                                                        ⁢                            t                                                    }                                                                                                      ]                                                                                        Expression        ⁢                                  ⁢                  (          2          )                    
Jn in Expression (2) is an nth-order Bessel function. The J0(k) term represents the optical carrier component, the J1(k) term the first-order sideband component, the J2(k) term the second-order sideband component, and the J3(k) term the third-order sideband component. The following description will focus only on the term of the first-order sideband component J1, and the second-order and subsequent components are ignored. The term of the first-order sideband component J1 can be extracted from the last line of Expression (2) and expressed as shown in Expression (3).J1 component=−γEJ1(kα)[sin ω0t cos ω1t]=−γEJ1(kα)[sin(ω0+ω1)t+sin(ω0−ω1)t]  Expression (3)
The photoelectric field of the optical carrier passing through the other optical waveguide can be represented by EB(t) in the first line of Expression (4). The photoelectric field of an optical intensity-modulated signal that can be obtained through an optical intensity modulation of the optical carrier can be represented by EB(t) in the fourth line of Expression (4). Herein, Vb(t) is the RF (radio frequency) signal voltage applied to the other optical waveguide, ka is the amplitude of the RF signal, and VB is the amount of phase to be given to the optical carrier by the DC bias voltage applied to the other optical waveguide.
                                                                                          E                  B                                ⁡                                  (                  t                  )                                            =                                                (                                      1                    -                    γ                                    )                                ⁢                E                ⁢                                                                  ⁢                cos                ⁢                                                                  ⁢                                  ω                  0                                ⁢                t                                                                                                                          V                  b                                ⁡                                  (                  t                  )                                            =                                                k                  b                                ⁢                sin                ⁢                                                                  ⁢                                  ω                  1                                ⁢                t                                                                                                        V                B                            =                              π                2                                                                                                                          E                  B                                ⁡                                  (                  t                  )                                            =                                                (                                      1                    -                    γ                                    )                                ⁢                E                ⁢                                  {                                      cos                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                                              V                            b                                                    ⁡                                                      (                            t                            )                                                                          +                                                  V                          B                                                                    )                                                        }                                                                                                        =                                                (                                      1                    -                    γ                                    )                                ⁢                E                ⁢                                  {                                      cos                    ⁡                                          (                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                                              k                            b                                                    ⁢                          sin                          ⁢                                                                                                          ⁢                                                      ω                            1                                                    ⁢                          t                                                +                                                  π                          2                                                                    )                                                        }                                                                                                        =                                                -                                                            (                                              1                        -                        γ                                            )                                        2                                                  ⁢                                  E                  ⁡                                      [                                                                                                                                                      sin                              ⁢                                                              (                                                                                                      ω                                    0                                                                    ⁢                                  t                                                                )                                                            ⁢                                                              cos                                ⁡                                                                  (                                                                                                            k                                      b                                                                        ⁢                                    sin                                    ⁢                                                                                                                                                  ⁢                                                                          ω                                      1                                                                        ⁢                                    t                                                                    )                                                                                                                      +                                                                                                                                                                                                          cos                              ⁡                                                              (                                                                                                      ω                                    0                                                                    ⁢                                  t                                                                )                                                                                      ⁢                                                          sin                              ⁡                                                              (                                                                                                      k                                    b                                                                    ⁢                                  sin                                  ⁢                                                                                                                                          ⁢                                                                      ω                                    1                                                                    ⁢                                  t                                                                )                                                                                                                                                                          ]                                                                                                                          =                                                -                                                            (                                              1                        -                        γ                                            )                                        2                                                  ⁢                                  E                  ⁡                                      [                                                                                                                                                      sin                              ⁡                                                              (                                                                                                      ω                                    0                                                                    ⁢                                  t                                                                )                                                                                      ⁢                                                          {                                                                                                                                    J                                    0                                                                    ⁡                                                                      (                                                                          k                                      b                                                                        )                                                                                                  +                                                                                                                                                                                                                                                                                                    2                                ⁢                                                                                                      J                                    2                                                                    ⁡                                                                      (                                                                          k                                      b                                                                        )                                                                                                  ⁢                                cos                                ⁢                                                                                                                                  ⁢                                2                                ⁢                                                                  ω                                  1                                                                ⁢                                t                                                            }                                                        +                                                                                                                                                                                                          cos                              ⁡                                                              (                                                                                                      ω                                    0                                                                    ⁢                                  t                                                                )                                                                                      ⁢                                                          {                                                                                                2                                  ⁢                                                                                                            J                                      1                                                                        ⁡                                                                          (                                                                              k                                        b                                                                            )                                                                                                        ⁢                                  sin                                  ⁢                                                                                                                                          ⁢                                                                      ω                                    1                                                                    ⁢                                  t                                                                +                                                                                                                                                                                                                                                                    2                              ⁢                                                                                                J                                  3                                                                ⁡                                                                  (                                                                      k                                    b                                                                    )                                                                                            ⁢                              sin                              ⁢                                                                                                                          ⁢                              3                              ⁢                                                              ω                                1                                                            ⁢                              t                                                        }                                                                                                                ]                                                                                                          Expression        ⁢                                  ⁢                  (          4          )                    
Jn in Expression (4) is an nth-order Bessel function. The J0(k) term represents the optical carrier component, the J1(k) term the first-order sideband component, the J2(k) term the second-order sideband component, and the J3(k) term the third-order sideband component. The following description will focus only on the term of the first-order sideband component J1, and the second-order and subsequent components are ignored. The term of the first-order sideband component J1 can be extracted from the last line of Expression (4) and expressed as shown in Expression (5).
                                                                                                              J                    1                                    ⁢                  component                                =                            ⁢                                                                                                       ⁢                                                -                                                            (                                              1                        -                        γ                                            )                                        2                                                  ⁢                E                ⁢                                  {                                      2                    ⁢                                                                  J                        1                                            ⁡                                              (                                                  k                          b                                                )                                                              ⁢                    cos                    ⁢                                                                                  ⁢                                          (                                                                        ω                          0                                                ⁢                        t                                            )                                        ⁢                    sin                    ⁢                                                                                  ⁢                                          ω                      1                                        ⁢                    t                                    }                                                                                                        =                            ⁢                                                -                                      (                                          1                      -                      γ                                        )                                                  ⁢                                                      EJ                    1                                    ⁡                                      (                                          k                      b                                        )                                                  ⁢                                  {                                                                                                                                          sin                            ⁢                                                          (                                                                                                ω                                  1                                                                +                                                                  ω                                  0                                                                                            )                                                        ⁢                            t                                                    +                                                                                                                                                              sin                          ⁢                                                      (                                                                                          ω                                1                                                            -                                                              ω                                0                                                                                      )                                                    ⁢                          t                                                                                                      }                                                                                                        =                            ⁢                                                -                                      (                                          1                      -                      γ                                        )                                                  ⁢                                                      EJ                    1                                    ⁡                                      (                                          k                      b                                        )                                                  ⁢                                  {                                                                                                                                          sin                            ⁢                                                          (                                                                                                ω                                  0                                                                +                                                                  ω                                  1                                                                                            )                                                        ⁢                            t                                                    -                                                                                                                                                              sin                          ⁢                                                      (                                                                                          ω                                0                                                            -                                                              ω                                1                                                                                      )                                                    ⁢                          t                                                                                                      }                                                                                        Expression        ⁢                                  ⁢                  (          5          )                    
Consider an optical intensity-modulated signal being obtained by combining together an optical phase-modulated signal produced by a phase modulation through one optical waveguide and another optical phase-modulated signal produced by a phase modulation through the other optical waveguide. The sideband component of the optical intensity-modulated signal can be obtained by adding together the sideband J1 component of Expression (3) and the sideband J1 component of Expression (5). In an ideal state where the light branching section 960 has no manufacturing error, or the like, and the light branching section 960 branches the optical carrier with a 1:1 ratio, the branching ratio γ will be 1/2. Assuming that ka=kb, the sum of the sideband J1 component of Expression (3) and the sideband J1 component of Expression (5) is such that the sin(ω0−ω1)t terms are canceled out by each other while the sin(ω0+ω1)t terms are not canceled out. For example, the sin(ω0−ω1)t term and the sin(ω0+ω1)t term represent the lower sideband component and the upper sideband component, respectively. Thus, there is obtained an optical intensity-modulated signal having a single sideband component (see FIG. 18A).
On the other hand, where the light branching section 960 has a manufacturing error, or the like, and the light branching section 960 cannot branch the optical carrier with a 1:1 ratio, the branching ratio γ will not be 1/2. Even if ka=kb, the sum of the sideband component J1 of Expression (3) and the sideband component J1 of Expression (5) is such that the sin(ω0−ω1)t components are not canceled out by each other. Thus, the unnecessary sideband component does not disappear but remains (see FIG. 18B).
If one sideband component remains, the optically-modulated signal is influenced by the wavelength dispersion as described above while being transmitted through the optical fiber. FIG. 20 is a diagram showing the relationship between the optical waveguide manufacturing error and the degree by which the unnecessary sideband component is suppressed. The horizontal axis represents, as the optical waveguide manufacturing error (waveguide error), the difference between the intensities of light beams inputted to the two optical waveguides divided by the sum of the intensities of the light beams inputted to the two optical waveguides in terms of percentage (%). The vertical axis represents the intensity ratio between the necessary sideband component and the unnecessary sideband component in terms of dB. As can be seen from FIG. 20, a 1% manufacturing error results in a degree of suppression of about 35 dB, and a 2% manufacturing error results in a degree of suppression of about 30 dB. Thus, a small manufacturing error results in a substantial deterioration in the degree of suppression.
Where light is intensity-modulated with a baseband digital signal having a low frequency, an optical intensity modulation such that one sideband component disappears is not typically performed, but an optical intensity modulation with both sideband components is performed. Where one sideband component is not made to disappear, the problem of the unnecessary residual sideband component does not occur.
With an external optical modulation section, there is a phenomenon called “DC drift” where a change in the temperature, or the like, also changes the optimal point of the bias voltage. Conventionally, when an optical intensity modulation is performed with a low-frequency signal, the bias voltage is controlled according to the change in the optimal point to thereby solve the DC drift problem. On the other hand, when an optical intensity modulation is performed with a high-frequency RF signal, both sideband components disappear at short intervals, whereby there is a greater need to perform an SSB optical modulation, or the like, such that one sideband component is made to disappear, as described above. In view of this, one possible way is to perform a bias voltage control to eliminate DC drift in order to solve the problem of a residual sideband component due to the waveguide manufacturing error. However, the problem cannot be addressed sufficiently with a bias voltage control.